With memory chips becoming increasingly integrated, a memory cell and the cell surface area thereof in which a capacitor is to be formed are becoming smaller. Subsequently, as a proper method for securing a Dynamic Random Access Memory (hereinafter, "DRAM") with a 25 fF (pico Farad)/Cell capacitance in such a small portion of a cell, several studies are being conducted for fabricating a thinner NO(nitrogen-oxygen) dielectric film into three-dimensional capacitor structures, such as a cylinder type capacitor, a fin type capacitor, a trench type capacitor, and applying Ta.sub.2 O.sub.5 (.epsilon.r=24) having a high dielectric constant (.epsilon.r) or a Perovskite structure material such as (Ba.sub.0.5,Sr.sub.0.5)TiO.sub.3 (.epsilon.r&lt;300) (hereinafter, "BSTO") or Pb(La,zr)TiO.sub.3 (hereinafter, "PLZT") to the dielectric film for forming a capacitor.
When used as a Perovskite type dielectric material, BSTO or PLZT exhibits a high dielectric constant due to the displacement occurring between the Ti and O atomic surfaces, and tends to exhibit ferroelectric properties. In recent studies, two directions have been investigated in applying a Perovskite structure BSTO or PLZT having a high dielectric constant to a capacitor dielectric film for a DRAM or Ferroelectric Random Access Memory (hereinafter, "FRAM").
One proposal is to form an electrode layer such as YBa.sub.2 Cu.sub.3 O.sub.7-x (hereinafter, "YBCO") or La.sub.1-x Sr.sub.x Co.sub.1 O.sub.3 (x=0.5) (hereinafter, "LSCO") on a substrate composed of a mono-crystalline, such as MgO or STO, which exhibits a proper lattice matching with BSTO or PLZT. High dielectric constant films of Perovskite structure are consecutively formed thereon into an epitaxial structure, which is comparable in bulk physical properties with a dielectric film having no grain boundaries.
The other proposal is to form a poly-crystalline high dielectric film on a lower side electrode layer composed of Pt/Ti/SiO.sub.2 or RuO.sub.2 /SiO.sub.2. The structure is focused on improving the dielectric and electrical properties of the dielectric film. The deposition conditions, such as temperature and oxygen partial pressure, are regulated instead of regulating a crystalline orientation direction thereof in relation with the lower side electrode.
According to an article in J. Appl. Phys., Vol. 76, No. 5, 1 Sep. 1994, whose disclosure is incorporated by reference, (001)-phase-oriented high dielectric film of a Perovskite structure has no grain boundaries therein exhibits a higher dielectric constant than those of a poly-crystalline structure, which results from the epitaxial structure. Since a (001)-phase orientation denotes a higher dielectric constant than those of other phase orientations, excellent electric properties and flat surfaced film are secured. Even with such excellent properties of the epitaxial structure, application thereof encounters difficulties in manufacturing memory chips adopting an epitaxial structure, and the properties are still under study.
Studies continue on a poly-crystalline high dielectric film which is applicable to memory chips. Leading examples for electrode layer materials being employed in a poly-crystalline structure are metallic electrode layers of Pt/Ti/SiO.sub.2 and conductive oxide films, such as RuO.sub.x, RuO.sub.2 /Ru. In recent years, Perovskite conductive oxide films, such as SRU(SrRuO.sub.3), are being considered as lower side electrode layer materials.
An article in Journal of Electronic Materials, Vol. 23, No. 1, 1994, whose disclosure is incorporated herein by reference, reveals that a Pt-related electrode causes electrical short circuits in a capacitor due to hillock generation during a high temperature process thereof and causes grain growth therein which produces difficulties in chip fabrication. Further, RuO.sub.2 or SRO enables formation of a high dielectric film with improved properties. In a poly-crystalline structure, as described above, a high dielectric film is deposited at a temperature below 650.degree. C., according to a report in Extended Abstracts of the 1991 International Conference on Solid State Devices and Materials, Yokohama, 1991, pp. 195-197.
However, the high dielectric film deposited on a substrate at such a low temperature has a tendency towards a certain crystal phase depending on the deposition conditions. Further, it is difficult to regulate the crystal phase therein, and there may occur an incomplete crystal formation therein. The dielectric film deposited thereon is composed of both a plurality of smaller grains and subsequently increased grain boundaries. An unstable phase may be formed therein resulting from boundary segregation between grains.
As described in Extended Abstracts of the 1994 International Conference on Solid State Devices and Materials, Yokohama, 1994, pp. 682-684, a BSTO(100) phase is preferred on a RuO.sub.2 (110) phase, and a PZT(Pb(Zr,Ti)O.sub.3)(100) phase is known to be formed preferentially on a Pt (111) phase within a certain deposition temperature. However, both phases are known to cause disadvantages such as a difficulty in forming a complete (001) phase Perovskite dielectric film and the generation of growth of other phases.
Further, although the upper surface of the high dielectric film is generally flat, the film itself has random crystal phases instead of predetermined ones, and interfaces are formed therebetween devoid of electrode layers or lattice matchings. Hence, interfaces having larger surface energy, as in high angle grain boundaries, are formed.
Therefore, there are several disadvantages with regard to the above-described dielectric film. Trapped charges occur along the interface between the electrode layer and grain boundaries within the high dielectric film. Thus the leakage current is increased while the TDDB (time-dependent direct breakdown) is decreased. Moreover, the high dielectric film, which is formed on an electrode layer and has a random poly-crystalline structure, exhibits a lower dielectric constant than that of an epitaxial structure or a poly-crystalline structure with a single phase orientation.